Recently, it has been examined by the 3GPP to construct a small cell (a CSG cell) by installing an indoor home base station (HeNB) in a cell (a macrocell) constructed by an ordinary base station (eNB) (see FIG. 11). The HeNB has a function of restricting terminal devices (UEs) which can access the HeNB, and a terminal device can connect to only an HeNB that it is permitted to access. Therefore, even if detecting an HeNB with a good reception quality, the terminal device cannot connect to the HeNB if the terminal is not permitted to access the HeNB (except for the case of emergency).
In order for the terminal device to obtain permission to access an HeNB, the terminal device needs to confirm cell identification information (a cell ID) included in notification information transmitted from the HeNB first. Each CSG cell has a group ID called a CSG ID, which is given to each group of CSG cells, and access permission to each group is given to the terminal device. As this cell ID, there are, for example, a physical cell ID (PCI) locally identifying a base station, a cell global ID (CGI) capable of uniquely identifying a base station among all base stations. The terminal device compares a list of accessible HeNB group IDs (an access permission list) notified from a network with a received cell ID in the notification information. If the detected cell ID of the HeNB is included in the access permission list, the HeNB is judged to be accessible.
As shown in FIG. 11, HeNBs are connected to a gateway apparatus for HeNBs (HeNB GW) and a mobility management apparatus (MME), and these apparatuses perform securement of network resources and management of NAS messages (such as position registration messages). Since the information of the access permission list is information about individual terminals, the HeNBs (assumed to be owned by individual households) do not have the information of the access permission list from the viewpoint of security. On the other hand, the MME can have the information of the access permission list because it is managed by an operator.
As described above, all CSG cells have cell identification information called a physical ID (PCI). However, there are not so many cells that can be identified by a PCI. Therefore, as shown in FIG. 12, there is a case where another CSG cell having the same PCI exists in one macrocell (a serving cell currently connected by the terminal device).
The terminal device measures the reception quality of a signal from a neighboring CSG cell. If the reception quality clears a predetermined condition (for example, in the case where the reception quality of the CSG cell is higher than the reception quality of the serving cell), a trigger for performing handover to the CSG cell (also called an event trigger) is caused. When this event trigger is caused, the terminal device notifies information about the CSG with a good reception quality (information about the reception quality and a PCI) to the eNB of the serving cell using a measurement result report. The eNB of the serving cell determines the handover destination CSG cell on the basis of the notified information (the information about the reception quality and the PCI) and transmits a handover request to the HeNB of the handover destination. In this case, however, if the above-stated PCI collision occurs, there is a possibility that the eNB of the serving cell transmits the handover request to the HeNB of a wrong CSG cell (this is called PCI confusion) as shown in FIG. 12.
For example, in FIG. 12, a terminal device measures the reception quality of the CSG cell on the left side of the figure and transmits a measurement result report to the eNB of the serving cell on the assumption of handover to this CSG cell (a target cell). However, the eNB wrongly sends a handover request to the HeNB of another CSG cell having the same PCI (the CSG cell on the right side of the figure). As a result, the terminal device fails in handover to the target cell (the CSG cell on the left side of the figure).
As a measure for such PCI confusion, there has been conventionally proposed a method in which a CGI (in addition to a PCI) is used as the cell identification information, for example, a method in which a CGI detected from notification information about a CSG cell is notified to a serving cell with the use of a measurement result report to uniquely identify the CSG cell. However, it takes more time to detect a CGI (in comparison with a PCI). Therefore, a terminal device notifies a PCI and a reception quality to a serving cell by a first measurement result report first, and, after detection of a CGI, notifies the CGI to the serving cell by a second measurement result report (for example, see Non Patent Literature 1).
Specifically, as shown in FIG. 13, the terminal device measures the reception qualities of surrounding CSG cells (S100). When finding CSG cells with a good reception quality (when an event trigger occurs) (S101), the terminal device notifies the PCIs and measured reception qualities of the CSG cells with a good reception quality, to the eNB using a first measurement result report first (S102). The eNB of the serving cell selects one handover destination candidate CSG cell from the first measurement result report received from the terminal device, and transmits measurement setting information which includes setting information required to detect the CGI of the CSG cell, to the terminal device (S103). The terminal device interrupts reception from the serving cell on the basis of information (information for interrupting data transmission such as a gap and DRX) included in the measurement setting information received from the serving cell, and detects the CGI of the CSG cell specified as a target cell (S104), and notifies the detected CGI to the serving cell using a second measurement result report (S105).
However, in the conventional method, settings for a gap (a period during which data is not transmitted/received), DRX (discontinuous reception) and the like are made by the terminal device on the basis of the measurement setting information at the time of detecting a CGI, and data transmission from the serving cell to the terminal device is interrupted (thereby, the terminal device can detect the CGIs of surrounding CSG cells). Therefore, if CGI detection is frequently performed, interruption of communication between the serving cell and the terminal device and decrease of throughput are caused (though, if frequency of CGI detection is low, it does not matter much).
Especially, HeNBs assumed to be installed in households are often arranged freely to some extent (in comparison with an eNB arranged by an operator in consideration of an installation place). For example, multiple HeNBs may be adjacently arranged near the cell edge of a macrocell (see FIG. 14). If a terminal device passes through such an area, the reception quality of the macrocell is low (because the terminal device is located at the cell edge of the macrocell), and there is a strong possibility that an event trigger for a nearby CSG cell occurs. Furthermore, in this case, since the multiple HeNBs are adjacently arranged, there is a possibility that an event trigger for another CSG cell is caused only by the terminal device moving a little. That is, in such a case, there is a possibility that event triggers for the multiple CSG cells continuously occur.
For example, it is assumed that three CSG cells exist around a terminal device located at a macrocell edge, and event triggers for these three CSG cells continuously occur (see FIG. 14). In this case, as shown in FIG. 15, when an event trigger for the first CSG cell (CSG1) occurs (S110), the terminal device notifies the PCI and reception quality of the CSG cell (CSG1) to the eNB by a first measurement result report (S111). When receiving this first measurement result report, the eNB returns measurement setting information for detecting the CGI of the CSG cell (CSG1) to the terminal device (S112).
Next, when an event trigger for the second CSG cell (CSG2) occurs (S113), the terminal device notifies the PCI and reception quality of the CSG cell (CSG2) to the eNB by a first measurement result report (S114). When receiving this first measurement result report, the eNB returns measurement setting information for detecting the CGI of the CSG cell (CSG2) to the terminal device (S115).
Furthermore, when an event trigger for the third CSG cell (CSG3) occurs (S116), the terminal device notifies the PCI and reception quality of the CSG cell (CSG3) to the eNB by a first measurement result report (S117). When receiving this first measurement result report, the eNB returns measurement setting information for detecting the CGI of the CSG cell (CSG3) to the terminal device (S118).
Then, first, on the basis of the measurement setting information for the first CSG cell (CSG1), the terminal device detects the CGI of the CSG cell (CSG1) (S119), and notifies the detected CGI to the serving cell using a second measurement result report (S120). Next, on the basis of the measurement setting information for the second CSG cell (CSG2), the terminal device detects the CGI of the CSG cell (CSG2) (S121), and notifies the detected CGI to the serving cell using a second measurement result report (S122). The same goes for the third CSG cell (CSG cell 3).
As described above, when event triggers for multiple CSG cells continuously occur, measurement result reports are frequently transmitted from a terminal device to an eNB, and wireless resources are wasted. Furthermore, if CGI detection is frequently performed, communication is frequently interrupted.